The present disclosure relates to a device and a method for distance measurement for a laser processing system, and relates to a laser processing system. The present disclosure relates in particular to a laser welding head with an optical coherence tomograph.
In a laser deep-welding process, a vapour capillary arises during a welding process along the beam axis of the processing beam, which vapour capillary is also called a “keyhole” and is surrounded by liquid melt. The depth of the vapour capillary is related to the depth of the weld seam or welding penetration depth. The welding penetration depth is an important parameter, since a depth that is too small can lead to a lack of strength of the weld joint. In contrast with this, a depth that is too great can give rise to full-penetration welding, as a result of which the weld seam is visible on the rear side.
A measuring beam can be used to determine the depth of the vapour capillary or the welding penetration depth, which measuring beam is directed into the vapour capillary. Both the size and diameter of the measuring beam on the workpiece surface and also the lateral incident position are decisive for an error-free measurement of the depth of the vapour capillary or the welding penetration depth. The incident position of the measuring beam can be adjusted by deflecting the measuring beam. Since the measuring beam no longer runs on the optical axis when the beam is deflected, the measuring beam experiences image defects (aberrations), which enlarge the diameter of the measuring beam in the region of the focus. The measurement of the depth of the vapour capillary or the welding penetration depth may thus be inaccurate or bound up with errors.
Use may be made of mirror optics (e.g. galvo-scanners) for the seam deflection, such as are described in publications US 2016/0039045 A1, DE 10 2013 225 108 A1, EP 1 977 850 B1 and DE 10 2014 113 283 B4. Mirrors have two disadvantages for the positioning of the measuring beam onto the vapour capillary. On the one hand, small angular changes of the mirror cause large positional changes of the measuring beam on account of the reflection law. On the other hand, a plane mirror does not influence the measuring beam, apart from directional changes. In other words, aberrations which arise for example in the focusing lens system with a non-axial passage of the measuring beam cannot be compensated for by the plane mirror. When use is made of galvo-scanners, F-Theta lenses can be used as a focusing lens system, which ensure that the measuring beam always retains the same diameter independently of the deflection. However, these lenses are expensive and are usually optimised only for one wavelength. The high-energy processing beam and the measuring beam, however, often have different wavelengths.
Furthermore, a device for measuring the depth of a welding seam in real time during the welding or jointing of a workpiece by means of radiation is known from WO 2016/062636 A1. For the lateral adjustment of the focal position of the measuring light beam, the exit/entry face of the optical waveguide can be displaced with respect to the collimator lens transversely to its optical axis. Drives with a high degree of precision are required for this.